I Fail to see where he tried to make you look bad... it's a little bit sad...
Some people change hardware very often, it's quite common in the gaming world, so they buy and sell Pcs, and it's not a business, so asking for a professional website or a "proof of claim" is a bit extreme in my opinion.
I know from experience that not everyone is honest in this world but taking everyone as a dishonest person from the beginning seems like a strange way of doing business and do not seem to me a good long term policy.
I have a suggestion, however regarding the activation policy, maybe you can do what other companies do : ie give the possibility to a user to deactivate a computer when uninstalling the product, so you'd get the 3 activations thing, and when the computer is sold it is deactivated (so the software won't run on it) then reactivate the new computer.